Public address and speech reinforcement systems are used to amplify and broadcast voice signals. In these systems, microphones are placed close to the performers and the microphone signals are amplified, processed and fed to amplifiers and loudspeakers for broadcasting. In such systems, the loudspeaker signals couple back to the microphones, and if the gain is too high, the system can become unstable. This feedback between the loudspeakers and microphones is minimised by using directional microphones and having the microphones close to the performers to maximise the signal level.
An assisted reverberation system is used to improve and control the acoustics of a concert hall (auditorium). There are two fundamental types. The first is the in-line system, in which the direct sound produced on stage by the performer (s) is picked up by microphone(s), processed by feeding it through delays, filters and reverberators, and broadcast into the auditorium from several loudspeakers which may be at the front of the hall or distributed around the walls and ceiling. In an in-line system acoustic feedback (via the auditorium) between the loudspeakers and microphones is not required for the system to work (hence the term in-line). The PA and speech reinforcement systems described above are simple examples of in line systems.
The second type of assisted reverberation system is the non-in-line system in which a number of microphones pick up the reverberant sound in the auditorium and broadcast it back into the auditorium via filters, amplifiers and loudspeakers (and in some variants of the system, via delays and reverberators—see below). The rebroadcast sound is added to the original sound in the auditorium, and the resulting sound is again picked up by the microphones and rebroadcast, and so on. The non-in-line system thus relies on the acoustic feedback between the loudspeakers and microphones for its operation (hence the term non-in-line).
In turn, there are two basic types of non-in-line assisted reverberation system. The first is a narrowband system, where the filter between the microphone and loudspeaker has a narrow bandwidth. This means that the channel is only assisting the reverberation in the auditorium over the narrow frequency range within the filter bandwidth. An example of a narrowband system is the Assisted Resonance system, developed by Parkin and Morgan (1) and used in the Royal Festival Hall in London. The advantage of such a system is that the loop gain may be relatively high without causing difficulties due to instability. A disadvantage is that a separate channel is required for each frequency range where assistance is required.
The second form of non-in-line assisted reverberation system is the wideband system, where each channel has an operating frequency range which covers all or most of the audio range. In such a system the loop gains must be low, because the stability of a wideband system with high loop gains is difficult to maintain. An example of such a system is the Philips MCR (‘Multiple Channel amplification of Reverberation’) system [2,3], which is installed in several concert halls around the world, such as the POC Congress Centre is Eindhoven. It has been shown [3] that the power gain due to the MCR system is given by                               P          MCR                =                  1                      1            -                                          α                MCR                2                            ⁢              N                                                          (        1        )            where αMCw is the loop gain and N is the number of microphone, loudspeaker channels. The reverberation time is increased by the same factor.
An improved wideband non-in-line assisted reverberation system has been described in Pct Patent application NZ93/0041[4-6]. In this system (FIG. 1) N microphones pick up the sound in the primary room and the microphone signals are fed into a secondary room and are reverberated and scaled by the loop gain before being fed back into the primary room. In practice the secondary room is replaced with a reverberation matrix. This improved system allows the apparent volume in the primary room to be altered independently of the apparent absorption. The improved system shall be denoted VRA (Variable Room Acoustics). The power gain introduced by the system is given by                               P          VRA                =                  1                      1            -                                          α                VRA                2                            ⁢              N                                                          (        2        )            where αVRA is the loop gain. The expression contains the square of the number of channels N, compared with N in the MCR system. This is due to the fact that the primary and secondary rooms have an effective power gain of N. Both systems thus have the same mean power gain for                               α          VRA                =                              α            MCR                                N                                              (        3        )            The Philips system provides a reverberation time boost which is equal to the power gain. As a result, the reverberation boost is limited by the maximum attainable power gain before instability. The VRA system, however, allows the reverberation time to be boosted over and above the power gain increase by controlling the reverberation time of the secondary room (while holding its gain constant). It has been shown [6] that the VRA system gives a reverberation time boost of                                           T            ass                                T            1                          =                  β                                    (                                                1                  +                  β                                2                            )                        -                                                                                (                                                                  β                        -                        1                                            2                                        )                                    2                                +                                                      βα                    VRA                    2                                    ⁢                                      N                    2                                                                                                          (        4        )            where Tαss is the assisted reverberation time, T1 is the unassisted reverberation time in the primary room and β is the ratio of the secondary room to primary room reverberation time. The gain in RT is equal to the power gain (equation 2) for β=0, (the equivalent MCR case), and it rises monotonically from the value as β increases.
The main difficulty with non-in-line systems is that they can become unstable, due to the feedback between the microphones and loudspeakers. The problem is minimised by using a large number of channels and keeping the loop gain in each channel low. For example the Philips system typically uses between 60 and 100 channels. However, even with large numbers of channels and low loop gains the sound in the hall can sound ‘coloured’. In a natural room, the sound decay at any position in the room consists of the sum of an infinite number of room modes. Typically all of these room modes have the same or similar decay rates, and as a result the decay in dB is linear [7]. In a non-in-line assisted reverberation system, this similarity of decay results is reduced. As a result, some room modes have longer decay rates, and after a certain time those room modes with the longest decay times dominate the sound of the decay. The decay is perceived as containing one or more ‘ringing tones’, or being ‘coloured’. Those modes which produce ringing tones occur at frequencies where the loop gain through the system is high and where the phase is a multiple of 2π. The feedback at such frequencies is thus positive.
In-line systems attempt to minimise the feedback between loudspeakers and microphones, but the problem is never eliminated completely. Thus, the problem of colouration also occurs in in-line systems.
The improved non-in-line VRA system described in PCT patent application NZ93/00041 provides an increase in the reverberation time over previous systems for the same loop gain. However, the loop gain in the system is more complex, due to the fluctuating frequency response of the secondary room matrix. As a result, the improved system will produce a higher degree of colouration than the MCR system for the same loop gain (equation 3). It would therefore be desirable to design a reverberation matrix that has a low degree of fluctuation in its frequency response.
In the same manner, an in-line system that utilises a reverberator will have a greater propensity to become unstable since the reverberator produces a fluctuating loop gain that at some frequencies is higher than the loop gain without the reverberator. Again, a reverberator with a lower degree of fluctuation in its frequency response will reduce the problem.
In the ease of a single microphone, reverberator and loudspeaker it is obvious that a low degree of fluctuation requires that the reverberator have a constant magnitude at all frequencies, ie it is allpass. Most reverberator structures do not produce such a response, however, since they consist of a parallel connection of comb filters [8]. Allpass comb filter sections are often used following these parallel connections of comb filters to increase the echo density [8,9]; however, it is not likely that a series connection of allpass comb filters alone would produce a reasonable reverberation because the configuration does not simulate a simple sum of decaying exponential modes as occurs in a room.
In the multidimensional case the definition of ‘low fluctuation’ is more complicated since the reverberation matrix contains N2 transfer functions and each output is the sum of N input signals transmitted through N transfer functions.